Suggest using hex hash instead of decimal hash for extension versioning

According to postgres docs, seems like we can use any string for the extension version. Currently we're using decimal version of hash, but seems like using hex version would be more useful, since this is what most tools (e.g. gitlab and git command line) show, so it would be easier to check against.

https://www.postgresql.org/docs/12/extend-extensions.html#id-1.8.3.20.14

PostgreSQL doesn't assume anything about the properties of version names: for example, it does not know whether 1.1 follows 1.0. It just matches up the available version names and follows the path that requires applying the fewest update scripts. (A version name can actually be any string that doesn't contain -- or leading or trailing -.)